The short answer
To find how much mulch you need, multiply the bed area in square feet by the depth in inches, then divide by 324 to get cubic yards. One cubic yard covers 162 square feet at 2 inches deep or 108 square feet at 3 inches. A 20 × 5 ft bed at 3 inches needs 25 cubic feet, which is 13 bags of 2 cubic feet.
How the calculation works
Mulch volume is the area of the bed multiplied by the depth. In US units, the shortcut is:
cubic yards = square feet × depth in inches ÷ 324
The 324 comes from the fact that one cubic yard (27 cubic feet) spread 1 inch thick covers 324 square feet. To count bags, divide the volume in cubic feet by the bag size.
Worked example
A bed 20 ft long and 5 ft wide, mulched 3 inches deep:
- Area: 20 × 5 = 100 square feet
- Volume: 100 × 3 ÷ 324 = 0.93 cubic yards (25 cubic feet)
- Bags: 25 ÷ 2 = 12.5, so buy 13 bags of 2 cubic feet
Coverage at different depths
| Depth | Per cubic yard | Per 2 cu ft bag | Per 3 cu ft bag |
|---|---|---|---|
| 1 inch | 324 sq ft | 24 sq ft | 36 sq ft |
| 2 inches | 162 sq ft | 12 sq ft | 18 sq ft |
| 3 inches | 108 sq ft | 8 sq ft | 12 sq ft |
| 4 inches | 81 sq ft | 6 sq ft | 9 sq ft |
How deep to mulch
- Flower and shrub beds: 2–3 inches of shredded bark or wood chips.
- Around trees: 2–3 inches, spread wide toward the drip line, pulled back from the trunk.
- Topping up old mulch: 1 inch is often enough, since last year’s layer has broken down but not disappeared.
- Paths: coarse wood chips 3–4 inches deep.
Fine mulches mat down and can shed water when laid thick, so stay toward the lower end with them.
Tips
- Measure irregular beds in pieces. Split curves into rough rectangles and circles, work out each, and add them up. Or use the square footage calculator.
- Rake out old mulch that has matted before adding new, so water can get through.
- Buy an extra bag or two. Mulch settles by up to a quarter in the first few weeks.
Frequently asked questions
How much area does a cubic yard of mulch cover?
One cubic yard covers 324 square feet at 1 inch deep, 162 square feet at 2 inches, 108 square feet at 3 inches and 81 square feet at 4 inches.
How many bags of mulch make a cubic yard?
A cubic yard is 27 cubic feet, so it equals 13.5 bags of 2 cubic feet, or 9 bags of 3 cubic feet.
How deep should mulch be?
University extension services generally recommend 2 to 3 inches of organic mulch for beds, and no more than about 4 inches. Deeper layers can keep water and air from reaching roots.
How far should mulch be from a tree trunk?
Keep mulch a few inches away from stems and trunks. Piling it against the bark (a "mulch volcano") holds moisture against the trunk and encourages rot and pests.
Is bulk mulch cheaper than bags?
Usually, once you need more than two or three cubic yards. Bulk delivery adds a fee, so for small beds bags can still work out cheaper.
Sources
Related calculators
About this calculator. Every formula on this page is tested against worked examples from the sources above before publishing. Results are estimates for planning; confirm with your supplier or local code.
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